Termite or white ants
Q: My saguaro and the tree next to it seem to have a brown substance on the base. Are these termites? If so, how do you treat or is it too late?
A: The saguaro may have termites; it is hard to see from the photo. Typically desert encruster termites (Gnathamitermes perplexus) will colonize the outside of older saguaros where there are dead woody areas. They bring their mud habitat with them so you might see what looks like mud caked on the base of the cactus. Scrape a bit off and you might see the small white termites run for cover. Since termites are only after the dead wood, the live part of the cactus is safe from harm.
